1. Field of the Invention
The present invention relates to a method and apparatus for generating wideband frequency coverage from a narrowband voltage control oscillator, and in particular to a programmable frequency synthesizer.
2. Discussion of Related Art
Many applications, such as radio and television (TV) tuners, cable modems, and test equipment, require receivers that are able to tune over a wide frequency band, for example, from approximately 50 Megahertz (MHz) to 1000 MHz, or over multiple frequency bands. One conventional method to generate wide or multiple frequency bands is to include multiple voltage-controlled oscillators in the receiver. For example, the receiver may be provided with a number of oscillators, each tuned for a different frequency range, and a switch may be used to select the output of one oscillator at a time, depending on the desired frequency range at the time. Alternatively, the receiver may include circuitry to mix the outputs of two or more oscillators, for example one tunable oscillator and one fixed oscillator, to produce sum and difference frequencies. Multiple tuned circuits are then required to select either the appropriate sum or difference frequency. For example, in H.F. receivers, the output of a variable frequency oscillator tuning over, for example, a 500 kilohertz (kHz) range is mixed with the output of a crystal oscillator. Different crystals are switched in to select different frequency ranges. In another example, used in radio frequency (R.F.) instrumentation, Yttrium Iron Garnet (YIG) tuned oscillators, operating at several gigahertz (GHz), may be mixed to provide an output frequency range from a few Hertz to approximately 1000 MHz.
Another conventional solution is to use a single voltage-controlled oscillator and to select one of several resonant (tank) circuits for each range of frequencies needed. The value of one or more of the components of any of the resonant circuits may be changed by switching in more inductance or capacitance to change the frequency coverage. A problem encountered in these solutions is that the voltage-controlled oscillators require a high voltage, for example, 25 to 30 volts (V), for the voltage-variable capacitors (or varactor diodes) to tune over a large capacitance, and thus frequency range. This may be undesirable, particularly in applications where the other components of the receiver circuitry operate on a low voltage, for example, 3 to 5 V. Furthermore, the requirement of either multiple oscillators or multiple resonant circuits results in complex, expensive circuitry and bulky receivers.
Another conventional solution is to use direct digital synthesis (DDS) to generate the desired frequency range. However, current technology limits the operating frequency of DDS systems to below 300 MHz.
According to one embodiment, a programmable frequency synthesizer may comprise a voltage-controlled oscillator that provides a first signal, a regenerative frequency divider coupled to the voltage-controlled oscillator that receives the first signal and performs a fractional multiplication of a first frequency of the first signal to provide a second signal having a second frequency, and a programmable integer divider, coupled to the regenerative frequency divider, that receives the second signal and divides the second frequency by a predetermined integer to provide a third signal having a third frequency.
In another embodiment, a tunable receiver, operable over a wide frequency band, may comprise a programmable frequency synthesizer including a voltage-controlled oscillator, and adapted to generate an output signal having a frequency derived from a combination of the voltage-controlled oscillator, a regenerative frequency divider and a programmable integer divider. The tunable receiver may further comprise a phase-locked loop coupled to the programmable frequency synthesizer to tune an operating frequency of the voltage-controlled oscillator, and a loop filter coupled to an output of the phase-locked loop and to an input of the programmable frequency synthesizer. In one example, the programmable frequency synthesizer may include the voltage-controlled oscillator that provides a first signal, the regenerative frequency divider coupled to the voltage-controlled oscillator that receives the first signal and performs a fractional multiplication of a frequency of the first signal to provide a second signal having a second frequency, and the programmable integer divider, coupled to the regenerative frequency divider, that receives the second signal and divides the second frequency by a predetermined integer to provide the output signal having a third frequency.
According to another embodiment, a method for synthesizing an output signal having an output frequency range may comprise acts of generating, with an oscillator, a first signal having a first frequency range and performing a fractional multiplication of the first frequency range of the first signal to provide a second signal having a second frequency range. The method may further comprise an act of dividing the second frequency range with an integer divider to obtain the output signal having the output frequency range, wherein the output frequency range is larger than the first frequency range.
According to yet another embodiment, a programmable frequency synthesizer may comprise a voltage-controlled oscillator to generate a first signal having a first frequency range, and means for synthesizing, from the first signal, an output signal having an output frequency range that is larger than the first frequency range, said means being adapted to receive the first signal from the voltage-controlled oscillator. In one example, the means for synthesizing may include a regenerative frequency divider, coupled to the voltage-controlled oscillator, and an integer divider coupled to the regenerative frequency divider. In another example, the means for synthesizing may further include a quadrature generator, wherein the output signal is a differential output signal including at least one in-phase component and at least one quadrature component.